This study was motivated by a lack of information on long term trends in commercial property rents. It set out to compile a long term rent index for the office sector and focused on the City of London as an important location with a long history of office renting. Much literature has examined the operation of office markets during the last 30-40 years, often in terms of explaining cycles and short run adjustment processes. In contrast, office rents were studied here over nearly 150 years to explore whether they have grown in line with the national economy and what other factors have determined their behaviour over a longer time frame. Upon finding no suitable secondary sources for investigating these issues, the research utilised archived property management records held at the Guildhall Library, London, from which a database of 9,141 individual rental transactions was created. Using the repeat measures regression method, an annual office rent index was then estimated, spanning the period 1867-1959. This series revealed that, in real terms, City office rents did not experience growth over the period as a whole, but there were distinct phases of growth and decline that correspond well with the economic fortunes of this location. The rent index was then compared with other historical data for UK commercial real estate and extended to the present day using contemporary data series. This latter step enabled further quantitative analysis that indicates change in the relationships between rent, demand and supply that are consistent with structural change in the economy and property market. There are some limitations arising from the data and methods used. Nonetheless, the data and results reported in this thesis extend existing knowledge on the history of the office market and suggest several avenues for future research, which are outlined in the final chapter.